In late years, lubricants have been required to have fuel-saving characteristics and energy-saving characteristics from the view point of global environmental protection, and also long-drain characteristics from the view point of resource saving. Because of this situation, low-viscosity, thermal resistance, and evaporation resistance will be the challenges of lubricants in the future. Therefore lubricants, which are excellent in thermal stability and oxidation stability, evaporation resistance, and low-temperature fluidity, have been demanded.
In general, any of lubricants may cause a deleterious change in low-temperature startup and a decrease in power efficiency when the viscosity of base oil is too high. On the contrary, if the viscosity is too low, an increase in oil consumption and a bearing damage due to insufficient lubricity may occur. Further, the flow-point, which represents an index of the low-temperature fluidity of the base oil, is preferably −20° C. or less, but not particularly limited to.
In view of improvement of the thermal stability and oxidation stability, synthetic lubricants are preferable. Examples of base oil known in the art include poly-α-olefins, α-olefin copolymers, polybutenes, alkylbenzenes, polyol esters, dibasic acid esters, polyoxyalkylene glycols, polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, and silicone oils.
The synthetic lubricant has both good points and bad points depending on intended purposes. In many cases, poly-α-olefin is used in consideration of thermal stability and oxidation stability as well as low viscosity. However, the conventional poly α-olefins include many isomers even in the saturated aliphatic hydrocarbon compounds having the same molecular weight, so any particular component (isomer) cannot be taken out by any purification process such as distillation. Thus, even a synthetic oil with a certain viscosity becomes a mixture of a high-volatility component and a low-volatility component. When such a saturated aliphatic hydrocarbon compound is used as a lubricant, the high-volatility component will vaporize at first to cause an increase in viscosity of the lubricant in a machine at work. Therefore, the conventional poly-α-olefins cause frequent repetition of oil change before the maintenance time. The lubricant may be ideally configured to provide oil that allows a mechanical maintenance and a change of oil to be carried out at the same time or to provide oil that does not require any oil-change.
Further, with respect to compressor oils used in compression equipment and so on, in consideration of resource saving and non-pollution, rotary air compressors have been widely used because of their high efficiencies, small oscillations, and small noises, in place of the conventional reciprocating air compressors. The lubricating condition of the rotary air compressor, such as the contact of the lubricant with high-temperature and high-pressure air, is more stringent compared with the reciprocating type. In addition, the recent rotary air compressors have been downsized more than before, and their opportunities to be operated under bad ambient environment such as under oxidative gas (e.g., SOx or NOx) atmosphere or cutting-oil mist atmosphere have been increased. In such cases, sludge arises in the oil. The sludge attaches to the inside of the device and causes a filter blockage in an extremely short time, so the operation of the device may become impossible. Therefore, oil with a high sludge resistance under such an environment has been demanded.
As a process for improving the sludge resistance, in general, there are (1) a process of mixing a base oil having a high solubility of the generated sludge, such as alkylbenzene, alkyl naphthalene, or ester oil, with mineral oil; and (2) a process of adding a detergent-dispersant to mineral oil. However, as a process of most advantageously preventing the generation of sludge by the oxidation stability of base oil itself, a synthetic saturated aliphatic hydrocarbon compound has come to be used as the base oil of compressor oil in stead of mineral oil. In this case, the hydrogenation products of α-olefin oligomers using BF3 catalyst are widely used now. However, this manufacturing process cannot control the molecular distribution of oligomer and generate a myriad of isomers of each of compounds having the same degree of polymerization. Therefore, the product obtained by oligomerization of α-olefin with BF3 catalyst has a large amount of evaporation loss because of an extended boiling-point range with a difficulty in purification. Therefore, the development of a new synthetic oil, which can take the place of such poly-α-olefin has been demanded in the art.
With respect to vacuum-pump oil, as another example, vacuum technologies have been widely used in the fields of semiconductor production, solar cells, aircrafts, automobiles, and opto-electronics. In order to implement these technologies, conventionally, mechanical vacuum pumps such as a reciprocating vacuum pump and a rotary vacuum pump, and high vacuum pumps such as a rotary oil vacuum pump and an oil vacuum diffusion pump, have been widely known in the art. Further, a vacuum-pump oil based on synthetic oil has been used for lubricating, highly vacuating, and prolonging the life of movable parts of these vacuum pumps.
In late years, in association with the expansion of applicable field of the vacuum pump, thermal stability and an extensive degree of vacuum have been demanded and an improvement of vacuum-pump oil to satisfy such demands has been conducted. Further, in the field of application of the vacuum technology, the shortening of time leading to stable operation after activation of a vacuum pump has been demanded for productivity improvement. However, a vacuum-pump oil based on a saturated aliphatic hydrocarbon shows, in particular, poor low-temperature starting characteristics when used during the winter season or on cold district. In other words, such a vacuum pump requires a long time before leading to the stable operation. As a result, there are problems of a decrease in productivity of a target product and a difficulty in obtaining the stable quality of the product. Therefore, the development of novel synthetic oil has been demanded, in which the novel synthetic oil will have fluidity at low temperature and good thermal stability, and is capable of securing a high degree of vacuum and taking the place of the conventionally-used poly-α-olefin.
Further, rotational speeds of spindle motors used in electrical machineries and apparatuses, in particular, CD, DVD, HDD, polygon scanner, and the like have been increased year by year. At present, a high-rotational speed of 10,000 rpm or more is demanded. Conventionally, a rolling bearing typified by a ball-bearing has been used in each of these spindle motors. However, noncontact dynamic pressure fluid dynamic bearing or oil-impregnated sintered bearing have come to be used in terms of performance and cost effectiveness. The performances (mainly running torques) of these fluid dynamic bearing and oil-impregnated sintered bearing at high-speed revolution may be often defined based on the viscosity of lubricant to be used. The running torque at the high-speed revolution tends to lower as the viscosity decreases.
These lubricants should be prevented from undergoing the evaporative loss or destructive loss as far as possible because lubricity should be kept throughout the life without refilling, when these lubricants are once enclosed in a bearing mechanism.
The evaporative loss of hydrocarbon base oil typified by the general mineral oil increases as the viscosity (molecular weight) thereof decreases. Thus, both lowering the viscosity and lowering the evaporativity are hardly attained at the same time. In addition, aiming at this coexistence, the technology using ester, which is a polar compound, in a base oil has been known in the art.
However, for example, when a polar substance such as ester is used, disadvantages may occur such that various resin materials, for example, coating materials such as CD and DVD discs and a structural material such as a motor frame, are deformed or changed in color. In particular, in the case of each of CD and DVD that perform recording with optical signals, a coating resin should be prevented from optically clouding or deforming as far as possible.
In view of the above, there is an environment in that an ester-based oil solution having excellent characteristics cannot be substantially used. In contrast, in the case of CD and DVD discs and motor equipment using large amounts of resin materials, a lubricant using poly-α-olefin, which has lower evaporativities compared with that of mineral oil and excellent thermal resistance, has been conventionally used as a base oil.
The poly-α-olefin, which has been often used, is one obtained by cationic polymerization with BF3 catalyst to oligomerize α-olefin and then hydrogenation thereof. However, this method cannot control the molecular distribution of oligomer and many different isomers even from the respective compounds having the same degree of polymerization are generated. Therefore, the product obtained by oligomerization of α-olefin with BF3 catalyst has an extended boiling-point range with a difficulty in purification, so there is a disadvantage of a lot of evaporation loss.
As a process for manufacturing poly-α-olefin, a process for dimerizing a linear α-olefin using a Ziegler (organic aluminum compound) catalyst and then dimerizing a dimer using a Friedel craft catalyst is known (Patent Document 1).
The Ziegler catalyst of this Patent Document 1 leads to a low yield of a dimer even if a long-chain α-olefin is oligomerized, Therefore, an organic aluminum compound, which is a kind of the Ziegler catalyst, may be employed as dimerization catalyst. In this case, however, the content of vinyliden olefin in the dimer is low even though the dimerization of the organic aluminum compound advances. Thus, there is a disadvantage in that the dimer will lead to a low content of α-olefin tetramer even though it is subjected to dimerization and hydrogenation.
For the oligomerization of decene, acid catalyst (such as BF3) has been currently used and a decene trimer hydride with a low flash point has been produced while having a high viscosity (Patent Document 2). In addition, in late years, a dewaxing lubricant obtained by oligomerizing decenes has been introduced. However, even if kinematic viscosities can be coincident with each other, any lubricant having low-temperature fluidity with high flash point cannot be obtained in the same kinematic viscosity (Patent Document 3).
In addition, as a process for producing a decene oligomer, there is known that a decene oligomer having a number average molecular weight of 500 to 200,000 is produced using a metallocene catalyst, subsequently hydrogenated as needed, and used as a lubricant base oil (Patent Document 4). In this process, for example, the oligomerization of C10 (decene) leads to a decrease in a production ratio of C20, C30, and C40 in this order, and a high yield of C40 cannot be attained.    Patent Document 1: GB 961903 B    Patent Document 2: JP 10-504326 A    Patent Document 3: JP 2002-502436 A    Patent Document 4: JP 2002-518582 A